Earth Resources: Mineral Identification Introduction Animal, Vegetable, or Mineral? If you are like most Americans, at some time, you have played the guessing game “Animal, Vegetable, or Mineral?” The premise behind the game is that one individual receives a certain number of questions in which to guess what material object another person has chosen. Of course, the first question is the aforementioned one, as every object that we can think of is supposed to fall within one of these three broad classification schemes. Alas, this is not really true. One problem is that some objects fall into 2 or more of these categories (some viruses have a crystalline stage, some animals undergo photosynthesis, etc.). However, the idea behind the game is that everything made of matter will fall into a classification of either living or non-living, with minerals being the catch all for non-living. The other problem with the game is that a mineral is much more than just a non-living object. What it is exactly, though, will raise a debate amongst geologist. A check of different textbooks will find many different definitions for mineral. For the purposes of this activity, we are going to define a mineral as a substance that is naturally occurring, inorganic, crystalline in nature, and has a definite chemical make-up. The first of these criteria means that anything man-made is not considered a mineral. This is somewhat problematic, as mankind has developed ways of creating certain gemstones in the lab that are almost indistinguishable from their natural counterparts. For instance, industrially created diamonds are used for many different tools, such as diamond-tipped saw blades. The second criterion is not without its problems, too. Certain minerals such as graphite, diamonds, and calcium carbonate can and do have biological origins. Graphite and diamonds can come from plant matter. Calcium carbonate is the chemical that makes up seashells. By convention, they are usually included amongst minerals. The third and fourth criteria are less problematic. The fact that a mineral must have a crystalline structure eliminates all liquids. It also eliminates all glasses, as these are amorphous solids with no definite atomic arrangement. The chemical make-up does come with one caveat: some minerals are allowed to have substitutions of certain chemicals in their molecular structure. As an example, hornblende is a complex mixture of hydrous ferromagnesium silicate that can various proportions of calcium, aluminum, and sodium within it. These substitutions usually just change the color of the mineral and do not radically alter the other properties of the mineral. Fig. 1: Calcite crystal (USGS) Identification To accurately identify a mineral and be 100% certain, an individual would have to run a number of laboratory tests on a sample. They would have to run an X-ray diffraction analysis of the material to find out what its true crystalline shape is. A ground-up and prepared sample would have to be put through a chemical analyzer to determine its chemical formula. Both of these procedures would take a lot of time and money, and some of the sample would be destroyed in order to perform the analysis. For these reasons, we rarely run such test unless there is a great need to know the answer for sure.

Instead, most minerals are identified by their physical properties. Since minerals have a definite chemical make-up and crystalline shape, one can usually identify them things like their hardness, color, or crystalline shape. Some of the more common properties used to identify them are listed below. 1. Hardness – One of the most common properties upon which to base identification, this is a measure of the scratchability of a mineral. It is evaluated on the basis of the Mohs’ hardness scale, which identifies the hardness of certain keystone minerals on a 1-10 scale. The scale is 1 for talc, 2 for gypsum, 3 for calcite, 4 for fluorite, 5 for apatite, 6 for orthoclase, 7 for quartz, 8 for topaz, 9 for corundum, and 10 for diamond, the hardest substance known to humans. The principle behind the scale is that any substance that is higher in number is able to scratch a substance of a lower number. topaz will scratch quartz, fluorite will scratch gypsum, and diamond will scratch them all. For further reference, it should be noted that the average human fingernail is about a 2 ¼, a copper coin is a 3 ½, a steel nail is about a 5 – 5 ½, and glass is about a 5 ½ – 6. To do the hardness test, you will sometimes need to use considerable force. You should try to minimize the scratching of the mineral by limiting the size of the mark. Further, you should wipe the mineral after the scratch test to make sure that it did indeed scratch, and that you are not just seeing powdered residue on the surface left behind by the device with which you performed the test. 2. Luster – This is the appearance of the mineral surface in reflected light. This test can be very hard to perform, as dirt on the surface or an uneven surface will skew results. The test is best carried out when you are looking at a large crystal face. The different categories are metallic (reflect a considerable amount of light and look like a metal surface), adamantine (brilliant, like a polished jewel), vitreous (glassy), resinous, pearly, silky, and earthy (dull, very little reflection). 3. Color – While this seems to be a very simple property, it is far from easy to use this property. Impurities can greatly change the color of a mineral. Dirt or other substances on the surface can also give a false reading. Color is also very subjective. What one person would call green, another might call grayish. This property is most reliable for metallic minerals, and fails a lot for transparent minerals. As an example, gold and iron pyrite often look Fig. 2: Gold (left) and iron pyrite (USGS) very similar in color (see Fig. 2). This is one reason why iron pyrite is often called “fool’s gold”. 4. Streak – This property is the color of the mineral residue when it is powdered. Amazingly, this property is usually much more reliable than color. To create a streak, one would usually use a mortar and pestle to crush a small sample. However, the most used tool for measuring this is to use a piece of white, unglazed pottery. Since pottery has a hardness of about 6, this tool is unusable for minerals that have hardnesses of 6 and greater. 5. Cleavage – This is the tendency of a mineral to split along certain planes. A great example of a mineral that has excellent cleavage would be mica, which cleaves along flat planes to give very thin sheets. Other minerals such as halite will have several different faces upon which they will cleave, while some other minerals such as quartz have no cleavage (and yes, geologist are know to make bad, sexist jokes during the discussion of this property). 6. Crystalline shape – This is the geometric pattern that a lone crystal of the mineral will have. To see this pattern, though, the crystal needs to be reasonably large and not convoluted by many crystals growing over one another. Oftentimes, all that one sees is just a face or two of the crystal. This might be enough if the shape of the crystal is simple. 7. Fracture – This is the shape of a mineral when it is broken. This occurs for minerals like quartz that do not have cleavage. The different types of fractures are conchoidal (concave breakage reminiscent of glass), splintery, or uneven. 8. Specific gravity – This is the density of the mineral compared to water (1 gm/cm3). Most minerals will have a specific gravity in the 2.5-3.5 range. Some, such as the natural metal ores and few other minerals rich in metals, will have specific gravities much higher than this. Others, such as halite and gypsum, will be much less than this. To determine this property, one needs a graduated cylinder with water in it and a mass scale. Putting the mineral in the graduated cylinder will tell one the volume of the mineral by the amount of water it displaces. Putting the mineral on the scale will give its mass, which when divided by its volume in cubic centimeters, gives its specific gravity.

There are other specialized properties that exist that will identify one or two minerals. Magnetism is a property that quickly identifies magnetite or loadstone. Taste can be very useful in identifying halite, although one can get very sick of licking every transparent mineral in their collection hoping to find it. Calcite has the unusual property of birefringence, which means that unpolarized light travelling through it will be bent at two different angles. In other words, light passing through clear calcite will produce two different images. Additional Reading The following link goes to a USGS website that discusses many of the common minerals that we encounter in our everyday lives. Links to information are provided that give detailed descriptions of the minerals, as well as listing the more common uses for them and the locations of mines within the U.S.

Topic: Minerals and Materials Summary: Contains information about different minerals, their uses, and their origins Link: http://resourcescommittee.house.gov/subcommittees/emr/usgsweb/ USGS

Activity For this activity, we are going to try to identify ten different minerals from their properties. There is an attached listing of the major properties of the most commonly found minerals. Use it and any other resources you might have to identify the ten minerals, and list your findings on the sheet below. In order to help you by giving you a little practice with mineral identification, we suggest the following virtual identifier: http://facweb.bhc.edu/academics/science/harwoodr/Geol101/Labs/Minerals/ Your instructor will provide you with up to 10 different mineral samples for identification. To test for hardness, you will also be provided with a copper plate or penny, a steel nail, and a glass plate (you can provide your own fingernail). You will also be given a piece of unglazed porcelain tile to use as a streak plate and a magnet to test for magnetism. You might also be provided with a weak hydrochloric acid solution, depending upon the discretion of the instructor. Using these simple tools and your powers of observation, you should be able to identify the minerals. References General Geology of the Western United States – A Laboratory Manual by Bassett and O-Dunn, pp. 2-18, Peek Publications, Palo Alto, CA, 1980.

Activity Sheet Mineral Identification

ESA21: Environmental Science Activities Name:

# 1

2

3

4

5

6

7

8

9

10

Mineral

Identifying Characteristics

DESCRIPTIVE MINERAL TABLE Minerals with Metallic Luster Name and Composition Graphite C Molybdenite MoS2 Galena PbS Native Copper Cu

Hardness

Color

Streak

Features

1

Silver gray Blueishgray Silvergray Copperrose

Black

Native Gold Au

2½-3

Gold, whitegold, rose

Same as color

Native Silver Ag

2½-3

Silverwhite

Silverwhite

Bornite Cu5FeS4 Chalcopyrite CuFeS2

3

Rose to brown Brassyellow

Gray-black

Marks paper like a pencil, greasy feel, light in weight. One perfect cleavage. Soft, flexible, shiny plates (one perfect cleavage), often with hexagonal outline. Marks paper. Cube or octahedron crystals, cubic cleavage, bright metallic luster, heavy. Copper-rose color on fresh surfaces; greenish-gray surface film where altered. Heavy and malleable. Rare crystals; usually in compact masses. Often has a pale green surface coating of malachite. Color varies with impurities. Extremely heavy. May be gouged or sliced with a knife. Dissolves in aqua regia. Rare small crystals, and dendrites; nuggets in sedimentary deposits Tarnishes dark gray. Irregular fracture. Very heavy. Sectile. May occur as dendrites (see Gold) and wires in calcite and other minerals. Usually tarnished blackish-gray. lrridescent alteration coating common; brittle conchoidal fracture. “Peacock ore.” Often tarnished irridescent or chalky greenish-blue. Brittle, fairly soft, usually massive. Conchoidal fracture. Occurs in cubes with grooved faces, and pyritohedrons with 5-sided faces. Called “fool’s gold,” much lighter than true gold. Poor cleavage; fragile. Magnetic, granular or octahedral crystals common. No cleavage. Glittering flakes or wavy sheets. Streak is distinctive. Tendency to flake obscures true hardness.

1-1 ½ 2½ 2½-3

3½

Greenishgray Black Copperrose

Greenishblack

Pyrite FeS2

6

Light brassyellow

Black

Magnetite Fe304 Specular Hematite Fe203

6

Black

Black

6

Shiny steel-gray

Dark red

Minerals with Non-Metallic Luster Name and Composition Talc Mg3Si 4010 (0H)2

Hardness

Color

Streak

Features

1

White, pale green

Pearly

Kaolinite AI2Si2O5 (OH)4 Native Sulfur S

1–2½

White, cream

Earthy, dull

1½-2½

Yellow

Resinous, greasy

Extremely soft; soapy feel. Impurities may increase apparent hardness. One perfect cleavage; often in scaly masses. Soft, powdery texture. Smells earthy when damp. Usually in clay-like masses with dull appearance. Color, low hardness, light in weight. Detectable sulfur odor. Often in welldeveloped blocky crystals, or as a fine coating on volcanic rock.

Gypsum CaSO4 ~2H2O

2

Coloroless, white; sometimes pale orange

Vitreous, pearly

Borax Na2B4O7 •1OH2O Chlorite

2

White

Vitreous

2

Light to dark green

Vitreous to earthy

Carnotite K2(U02)2(VO4) 3H2O2

2

Canary yellow

Dull, earthy

Cinnabar HgS

2–2½

Cinnamon red

Adamantine to dull

Biotlte Mica K(Mg, Fe)3 AISi3O10 (OH)2 Muscovite Mica KAI2(AISi3 O10) (OH)2

2½

Dark brown, black

Vitreous

2½

Colorless, pale green

Vitreous to Pearly

Lepidolite Mica KLi2(AISi4O10) (OH)2 Halite NaCl

2½-4

Colorless, lilac, yellow

Vitreous to pearly

2½

Colorless, salmon, pastels

Vitreous to greasy

Asbestos Mg6Si4O10(OH)8

2½-3

Light green, light brown

Silky

Calcite CaCO3

3

Colorless, white; rarely pastel

Vitreous

Barite BaSO4

3

Colorless, white, blue

Vitreous

Bauxite

3–3½

White; usually stained with goethite

Earthy

Sphalerite ZnS

3½

Usually yellowbrown; also black, green, red

Adamantine to metallic

Soft, one perfect cleavage. Selenite is clear, satin spar is fibrous, alabaster is massive. Selenite may occur in large (to 1 m.) sword-like crystals; or in bladed groups incorporating sand and known as “desert roses.” Short, stubby crystals. Conchoidal fracture. Brittle, soft. Also in earthy, massive form. Green color and micaceous habit (one good cleavage). Flakes are not elastic like mica. Usually a coating or powder in sandstone or other rock; imparts a strong yellow color. Very radioactive. Hardness indeterminate. Color diagnostic. May appear almost metallic or in earthy, pinkish-red masses. Scarlet streak. Toxic. Occurs in six-sided mica “books” and as scattered flakes. Peels into thin flexible greenish-brown sheets along one perfect cleavage. Black mica. Occurs in mica “books” and as scattered flakes. Peels into thin flexible transparent sheets along one perfect cleavage. White mica. Lilac color is diagnostic. Often in granular masses of small mica “books.” Lavender mica. Easily dissolves in water. Often has stepped-down “hopper” faces. Cubic cleavage. Crystal masses or coating on other material. Long, threat-like fibers with silky sheen. The commercial variety is fibrous serpentine. Effervesces freely in cold dilute hydrochloric acid. Perfect rhombohedral cleavage. Doubly refracting. Frequently in rhombohedral crystals; hundreds of other forms known. May be fluorescent. Heavy for a non-metal. Often occurs as tabular crystals; such crystals in circular arrangement form “barite roses.” Perfect cleavage. Pea-sized round concretionary grains show color banding in cream, yellow, and brown. Actually a rock made up of various hydrous aluminum oxides. Light yellow streak in most color varieties. Heavy. Perfect dodecahedral cleavage; cleavage chunks often triangular in shape. Occurs as crystals, compact masses, and coatings.

Azurite Cu3 (C03)2 (OH)2 Malachite Cu2 C03(OH)2

3½-4

Azure blue and bright green, respectively

Dull or velvety

Dolomite CaMg (CO3)2

3½-4

White, yellow, pink

Vitreous to pearly

Fluorite CaF2

4

Vitreous

Colemanite Ca2 B6O11~ 5H2O Apatite Ca5(P04)3F

4½

Colorless, all pastels, deep purple Colorless, white

5

White, blue, brown

Vitreous

Scheelite CaWO4

5

White, yellow, brown

Vitreous

Goethite HFeO2

5-5½

Dark rusty brown, ochre yellow

Dull, earthy

Hematite (earthy) Fe203

5

Sub-metallic to earthy

Rhodonite MnSiO3

6

Dull brownish red to bright red Pink to deep rose

Hornblende

5½-6

Greenish-black

Vitreous

Auglte

6

Dark green

Vitreous to dull

Orthoclase Feldspar KAISi3O8

6

WhIte, pink

Vitreous

Plagloclase Feldspar NaAISi3O8 CaAl2Si2O8

6

WhIte, gray

Vitreous

Vitreous

Vitreous

Colors and association distinctive; both effervesce in hydrochloric acid. Azurite often in radiating masses. Malachite frequently in curved masses exhibiting color banding in shades of green. Slowly effercesces in cold dilute acid when powdered. Pale pink color is indicative. Often associated with calcite. Usually in rhombohedral crystals; perfect rhombohedral cleavage. Crystals often cubic or octahedral. Color banding common. Octahedral cleavage. Usually fluorescent in ultraviolet light. May be in stubby, glassy crystals, or in compact granular masses. Perfect cleavage. Will not scratch glass. Commonly in 6sided prisms. Green, Blue, Yellow. One poor cleavage. Will not scratch glass. Heavy. Fluoresces. Good cleavage, crystal faces may be grooved. Streak distinctive yellow-brown. Often spongy, porous or earthy; also bladed, fibrous. Also called limonite. Often occurs in cubes and pyritohedrons as an alteration of pyrite. Characteristic red-brown streak. Often earthy and too powdery for accurate hardness test. May be granular or oolitic. Crystals rare; no cleavage. Massive, dense or granular aggregates often have black veins. Color and hardness diagnostic. Blocky crystals, nearly 90° cleavage. Barely scratches glass. Shiny on cleavage faces; opaque; often splintery at edges. Usually massive; occasionally in chunky crystals. Two directions of cleavage at 124° and 56°. Stubby prismatic crystals. Usually duller and greener than closely related horn blende. Two cleavages at 87° and 93°, and uneven fracture. Two good cleavages. Will scratch glass. Wavy internal pattern and pink color distinguish it from plagioclase when present. May be massive, or in large, well-developed coffin-shaped crystals. Two good cleavages. Will scratch glass. “Record grooves.” Rectangular cleavage faces often seen In igneous rocks.

Spodumene LiAISi2O6

6½

Colorless, white, lavender

Vitreous

Olivine (Mg, Fe)2SiO4

6½-7

Olive green

Vitreous

Epidote Ca2(Al, Fe)3 Si3012 (OH) Quartz Family SiO2

6½-7

Light to dark green

Vitreous

7

Vitreous to greasy

Chalcedony (Quartz) (petrified wood, flint, chert, agate, jasper) Staurolite FeAl4Si2010 (OH)2

7

Colorless White Gray, brown Pink Purple Yellow Variable

7-7½

Brown

Vitreous

Tourmaline

7-7½

Black, brown, green, pink, blue yellow

Vitreous to dull

Garnet Fe3Al2 (Si04)3

7-7½

Brown, red; also purple, green, yellow, black, pink

Vitreous to

Beryl Be3Al3Si6 018

7½ - 8

Vitreous

Topaz Al2SiO4(OH,F)2

8

Colorless, white, pink, blue, light green, emerald green Colorless, white, golden yellow, light blue

resinous

Vitreous

Massive, dense, often bumpy masses; waxy surface. Color banded or mottled appearance common. Not wholly crystalline. May line rock cavities to form geodes, or replace organic material to “petrify” wood, shell or bone. Usually found as prismatic crystals; often twinned to form crosses. Crystal faces are pitted and rough. Cruciform twinning is diagnostic when present. Typically in elongated crystals with grooved faces and rounded triangular cross section. Common variety shiny black. Crystals often occur in parallel or radiating groups. No cleavage. Commonly in shades of red. Dodecahedral crystals have diamondshaped faces. Color and hardness aid identification. No cleavage. Transparent to opaque. Commonly pale green, and in 6-sided prisms with flat terminations. Harder than quartz. Poor cleavage. Distinct glassy prismatic crystals with perfect basal cleavage exhibit ing diamond-shaped cross section. Internal rainbows. Striations on crystal faces. Commonly in barrel-shaped 6-sided crystals, tapered or with flat ends. Extremely hard. No cleavage.

Gray, all Vitreous pastels, to greasy red, dark blue, brown 10 Colorless, Adamantine Octahedral crystals with greasy luster. Diamond C pastels, Hardest known substance. Two to greasy blue, yellow, directions of cleavage. gray, black From General Geology of the Western United States – A Laboratory Manual by Bassett and O-Dunn, pp. 618, Peek Publications, Palo Alto, CA, 1980. Corundum Al203

9

Waxy

Elongated prismatic crystals. Associated with lepidolite, tourmaline, beryl. Deep grooves often parallel long crystal faces. Perfect prismatic cleavage. Crystals often appear as glassy green beads, isolated or in masses. Color distinctive. Conchoidal fracture. Usually a dull avocado massive; crystals are dark green, with striations and well developed cleavage. Crystals are 6-sided prisms, often with terminations and steps perpendicular to crystal length. Conchoidal fracture; no cleavage. Crystals may be in clusters, or line cavities in rock; some weigh several hundred pounds.